


Let’s Get Knotty

by TellTaleKael



Series: Knotty Boys [1]
Category: Undertale (Video Game)
Genre: Alpha Cross, Alpha/Beta/Omega Dynamics, Babymaking, Breeding, Breeding Kink, Commission fic, Cross Sans - Freeform, Error Deals With Some Haphephobia But It’s All Good, Error Gets All The Love, Error Sans - Freeform, Error Sans/Cross Sans, Knotting, M/M, Mating Cycles/In Heat, Omega Error
Language: English
Status: Completed
Published: 2021-01-18
Updated: 2021-01-18
Packaged: 2021-03-17 00:06:53
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,201
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28839852
Author URL: https://archiveofourown.org/users/TellTaleKael/pseuds/TellTaleKael
Summary: “Error didn’t know if it had been his short, clipped replies to Cross’ texts, the fact that he cancelled their date last minute, or if Cross had always planned on stopping by, but he honestly wasn’t that surprised when a tear appeared in the anti-void and Cross walked through.”Error is in heat and his mate comes to help.
Relationships: Sans/Sans (Undertale)
Series: Knotty Boys [1]
Series URL: https://archiveofourown.org/series/2177424
Comments: 14
Kudos: 172





	Let’s Get Knotty

**Author's Note:**

> This was a commission by [Paranormal Aries](https://twitter.com/peach_gamer?s=21) over on Twitter. Featuring some good Error loving. Enjoy!

Error didn’t know if it had been his short, clipped replies to Cross’ texts, the fact that he cancelled their date last minute, or if Cross had always planned on stopping by, but he honestly wasn’t that surprised when a tear appeared in the anti-void and Cross walked through. He sunk lower into his beanbag, shoulders hunkered forward defensively. He knew the instant the scent hit Cross, making him take a single step back. Damn Alpha senses.

“Error?” Cross let his sword dissipate, eyelights zeroing in on Error’s nest of misery, beanbag molded to his body, surrounded by empty cups and snack wrappers. Error opened a portal and allowed the trash to fall through, dumping them on top of Underfell Sans’ head. The angry, “what the fuck-?” was cut off by the portal closing. Error pointedly didn’t look at Cross. 

“Why didn’t you tell me you were going into heat?” Cross asked, voice laden with concern. Error shrugged.

“It’s just preheat,” he said, voice low and rough from disuse. He had been clenching his jaw so tightly it ached. “‘M not far enough along to touch, so there was no reason to bother you.”

“Bullshit.” Cross dropped to the ‘floor’ over the anti-void in front of Error, legs and arms crossed. “I could have still been here, taking care of you. Have you eaten? Drank anything in a while?”

Error sank even lower. “I dunno. Can’t remember. Doesn’t matter, I can’t die, remember?”

“So? Doesn’t mean you should suffer.” Cross pulled a chocolate bar from his inventory, peeling the foil wrapper back. He broke a square off and waved it at Error. “C’mon, I went all the way to Underfell to get this for you. Have at least one bite?”

“Fine.” Error pulled himself up, leaning forward to snatch the chocolate with his tongues right out of Cross’ hand, though he was careful not to let them touch. Cross flushed ever so slightly, cheekbones stained with a fine dust of purple as he watched Error’s tongues. He must have been more affected by Error’s omega scent than he let on. 

The chocolate began to melt on Error’s tongues, sweet and rich. His nonexistent stomach clenched painfully, different than the cramping he had been experiencing since his preheat settled in. He was starving.

Cross was ready with another piece of chocolate when Error lurched forward. He carefully fed Error the whole bar, piece by piece, shivering when, in his eagerness, Error accidentally licked his hand. The fizzle of glitches against his tongue was offset by the heated look on Cross’s face. Definitely affected, then. Of course, Cross had never once pushed further or faster than Error was comfortable with, so once the chocolate was gone, it was replaced with a juice box. It soothed a throat he hadn’t even realized had been parched until that moment. He drank until the box crumpled, empty. Despite himself, he had to admit that he felt better. Less miserable. 

“Thanks,” he muttered, sitting back and tucking his face in his scarf. Now that he was no longer in so much discomfort, he could smell the rich, woodsy scent that wafted off of Cross, senses so much more sensitive during his cycle. It was a good scent, familiar and warm. If he was more of a romantic or poetic in any way, he would say it was the smell of home. 

Cross smiled at him, sockets softening. “Of course.” He sat back, looking at Error with too much fondness for Error to handle in that moment.

“Wanna watch something?” he asked, sliding off of the beanbag to sit beside Cross, close but not touching. It would take more time before he was ready for that, body working against its own needs. Cross respected the small distance, as always. 

“Undernovela?” Cross suggested with a smile. They had spent several dates just sitting around, watching the escapades of the monsters in that universe as if it were a tv show. It was Error’s favorite, after all.

Error opened a portal so they could peek into the happenings of that universe. Cross settled in, leaning back with his hands on the ‘floor’, clearly already confused but interested in the story. Error watched him for a long moment, the minute changes in his expression slightly more interesting than the show. When he did decide to tune in and pay attention, he placed a hand down next to Cross’. Slowly, he walked his pinkie over, curling it together with Cross’. A glance over told him that Cross held the same awed, happy expression that he had worn the first time Error had ever touched him. Sentimental fool. Error hid his yellow blush in his scarf once again, turning to face the portal. 

He didn’t quite remember when he had fallen for Cross. They worked together a few times under Nightmare. Error fought better at a range, while Cross was better at close quarters, so they were often paired together since they complemented one another’s style. Back then, he had thought of Cross as a simple alpha, a soldier who followed orders. Error’s own presentation was often hard to pin down, his glitches making his scent work strangely and his heats irregular at best. He didn’t remember ever being a stereotypical omega, but nothing about him had ever been normal. 

Cross, to his credit, had never looked down on Error after finding out his presentation, like some alphas would have. Which was good for him, because at the time Error might have strung him up and broken his bones for doing so. It also could have been because Nightmare made sure everyone knew he was an omega, powerful and controlling as he was, so Cross had plenty of experience with strong omegas, even if he was the youngest and less experienced of Nightmare’s crew. 

So it hadn’t been finding out about his presentation that had drawn Cross to Error. Nothing had changed for a while after that. No, it had been finding out that Error lived in the anti-void that had sparked their first connection. While Error didn’t mind the bland blank expanse of the anti-void, it reminded Cross far too much of his own universe, where he had been trapped before being brought into the multiverse. He insisted on visiting and bringing Error gifts. The table where his knitting supplies were stored, several blankets, even the beanbag had all been gifts from Cross to ‘liven up’ the place. Error had even gone so far as to hang up paintings by his strings to add more clutter, so Cross could visit without being assaulted by solid whiteness all around. 

At some point, quick visits turned into lingering ones, with more talking. Cross was nothing if not stubborn, worming his way into Error’s life and mind. The first time Error held his hand, he had made such an adorable expression that Error had to kiss him. He crashed immediately afterwards and took a while to reboot, but Cross stayed, waiting for him to wake back up. Their relationship had grown ever since. 

Cross laughed at something in the show, bringing Error back out of his musings. There was something breathtaking about the way Cross laughed, open and honest. Error moved slightly closer, resting his skull on Cross’ shoulder. When he didn’t crash, he relaxed, closing his sockets. With his stomach full and his alpha nearby, he felt secure enough to drift into a doze, consciousness fading slowly. 

***

Waking up was not nearly as pleasant as falling asleep. Error’s body itched uncomfortably, the contact of the clothing against his bones making it even worse. He shifted, realizing that he was still leaning against someone. He turned his skull, snuffling a little to catch that scent of outdoors. It made the hard knot in his chest ease slightly. “Alpha,” he mumbled, voice glitching a little. Cross inhaled sharply, and Error cracked his sockets open.

Cross was sitting almost ramrod straight, face purple. He still had his pinkie curled around Error’s, and seemed to be doing his best to breathe slowly, so as to not jostle Error’s skull.

His pants were tented, purple glow just barely visible through the dark fabric.

Error laughed. “Looks like someone’s excited.” 

Cross huffed. “You smell good.” 

Error considered. He was probably past the cusp of preheat by then, starting his full heat. His hand and skull both touched Cross without issue, no glitching or painful sparks. If they started slow, he would probably be alright. “Probably taste better.”

Cross’ eyelights shrank, hand coming up. It stopped before making contact with Error’s face, hanging midair. “You sure?”

Error nodded, leaning his skull to nuzzle against Cross’ hand. He swiped his tongues out for good measure, licking the joints of the alpha’s phalanges teasingly. “I’m ready.” 

It wasn’t the first time they had been together. Cross had seen him through a handful of heats already, and a few times outside of heat Error had been able to touch and be touched by his alpha. But every time, Cross was just as reverent as the first. He lifted Error’s hand, threading their phalanges together to make sure Error could handle it before moving on. Error squeezed lightly to show that he was okay, making Cross smile softly. He pulled Error in for a kiss, quickly losing himself trying to keep up with all five tongues. He always lost that battle, but he did his best, licking and suckling at Error, taking his time. Even with the bulge in his pants, he refused to rush, kissing Error until they were both panting for breath they didn’t need. When he pulled away, he laughed dazedly, mismatched eyelights sparkling. “You taste like chocolate,” he told Error. 

“Well, someone insisted on force feeding it to me.” Error let go of Cross’ hand so he could unwind the scarf from around his neck. Cross’ gaze zeroed in on the faint scar that curled over his neck, a curve of bite marks that Cross had put there himself that Error’s body accepted, not healing even though he had come away from far worse injuries with no marks at all. It was a claim from his alpha, and the sight of it alway made Cross’ scent flare possessively. This time was no different, and he lurched forward to tuck his face in Error’s neck, scenting him with deep breaths. 

“You smell so good,” he moaned. Error braced himself for the pain touch usually caused, but of course it didn’t come, at that point his heat need overpowering his haphephobia and recognizing his alpha. He relaxed in Cross’ arms, skull tilted back to offer up as much neck as possible. Teeth and tongue both explored his bones, scraping lightly and then licking to soothe the slight sting. Pressure and pleasure built in Error’s gut, replacing the cramping and discomfort that had been plaguing him. 

Hands finally slipped under Error’s shirt, phalanges curling around his ribs. The bone was sensitive, making him shiver as Cross stroked them lightly. “Can I take this off?” he asked breathily, and Error nodded, already feeling overheated. He wasn’t much cooler once it was off, but Cross being able to touch him more made up for the lack of breeze in the anti-void. Every stroke of his phalanges replaced the uncomfortable heat of his cycle with the burning of passion, leaving Error’s glitched soul pounding. 

Cross sat back suddenly, and a needy noise rose from Error’s throat before he could bite it back. He flushed, dark bones stained yellow, but Cross looked at him as if he were the most attractive thing he had ever seen. He scrambled to pull his coat off, spreading it out on the ‘floor’ before laying Error down on it. “I forgot blankets,” he explained as he fluffed the hood up to cradle Error’s skull. Error didn’t want to let go of Cross long enough for the alpha to go fetch one of the many blankets he had squirreled away in the area, so the coat was sufficient for the time being. It was soft, it smelled like Cross, and it felt like a hug, anyway, though he would never admit it. 

Cross leaned over Error to kiss him gently, as if breaking from the building passion so he could ground himself. Only then did he sit back, brushing his phalanges over the swell of Error’s iliac crest peeking out from under his shorts. Error’s magic had long since gathered in his pelvis, shapeless and throbbing. Cross could no doubt feel it reach up and curl around his phalanges, trying to coax him lower. He pinched the hem of Error’s shorts lightly. “May I?”

“You better,” Error shot back, kicking his slippers off and lifting his hips. His shorts soon joined his shirt somewhere on the ‘floor’ behind Cross, and he was finally bare. A hunger rose in Cross’ gaze, hands finding Error’s hips once more, teasing the bone there lightly. 

“You’re... the most gorgeous being I’ve ever seen,” Cross whispered, eyelights expanding as he looked upon Error. Error knew his body was one of the most interesting of their alternates, oddly colored, glitching and repairing itself in turn. He knew he looked good, but hearing it from Cross was... always nice. Made him feel good.

Reaching down, he circled his hands around Cross’ wrists, pulling them away from his hips. His ecto slid into place, dark blue and plush around his bones. Unlike Cross, who was more finely cut, his own body was similar to the original Sans’, soft and full. He was cute, far cuter than he had any right to be, really, but Cross never failed to admire him. And admire he did. His hands descended on Error’s ecto, massaging his tummy, hips, his thighs. Ecto always wasn’t as sensitive as his bones, and when he was in heat the touch didn’t really bother him at all. Cross knew that, and took advantage of the fact, stroking and squeezing every inch of Error’s pseudo flesh with fervor. It felt good to be touched without pain, and though Error would be happy to have Cross pet him for hours, there was a growing need pressing down on him. 

“I thought you were going to taste me,” he said a little breathlessly. He spread his legs, thighs parting to reveal his pussy, already wet in anticipation and desire. Cross let out a sound like a whimper, leaning down and pulling Error’s legs over his shoulders. 

He wasted no time in delving in, following Error’s scent to the source. The flat of his tongue swept over the length of Error’s slit to gather what slick had collected there, and then slipped inside seeking more. Error threw his head back against the fluff of Cross’ hood while its owner licked inside of him, stretching his inner walls and drinking his magic down with fervor. Cross’ woodsy scent was finally joined by Error’s, something spicy rich combined with the copper of old blood. The scents intermingled, combining into their own special one. It filled Error’s every breath, to the point that he could taste it on his tongues. 

Cross slipped his tongue free, moving up to lick at Error’s clit. Phalanges replaced his tongue, scissoring and stretching Error out even further. He pressed firmly against Error’s clit, and Error came with a shout, hands flying to grasp and press his skull closer as the pleasure crashed over him. 

His chest was heaving when Cross sat back, face stained blue and eyelights burning. His pants were uncomfortably tight around the bulge of his cock, but he focused first on Error as always, wiping his face and sliding up to kiss him again. Error allowed it for a moment, before he pushed Cross back.

“I want your knot,” he said. Cross froze.

“You what?” They had never knotted before, as a precaution. Error had no clue what his body would do if Cross’ seed caught... until recently.

“Sci ran some tests,” he explained, cutting his eyelights to the side. “Not saying it will happen, but if it worked... if you bred me... it should be okay.” 

Cross’ hand gently cupped Error’s chin, making him look back at the alpha. His expression was soft and full of wonder. “And if I did... you would be okay with it?”

Error blushed. “I w-would be. We’re already mated... there’s plenty of room between here and the castle...” He shrugged awkwardly.

Cross surged forward, lifting Error and flipping him over, depositing him on his hands and knees. There was a bit of fumbling, but Cross managed to shove his shorts down enough that his cock sprang free, pulsating and hard, knot already visible at the base. “I want to,” he panted, draping himself over Error’s back. “Want to knot you. Breed you. Fuck! Claim your womb as mine.” He lined his cock up with Error’s entrance, thrusting in sharply. 

Error arched back against Cross, vision going hazy for a moment with sharp pleasure. The words were utterly filthy, and hearing them fall from Cross’ mouth was sinfully good. His polite alpha, being pleasantly rough with him? He wanted nothing more.

“Then d-do it,” he challenged. 

Cross fisted the coat beneath Error, gripping tightly as he pulled out before slamming back in. The swell of his knot kissed Error’s entrance with every thrust, knocking trembling gasps from them both. Error’s walls clung to Cross as if trying to mold to the shape of his cock. It was messy and hurried, Cross fucking into him and Error rocking back to meet each thrust. Kisses rained down on his neck and shoulders, little hints of teeth and tongue that left him a shivering mess. He had never seen Cross like this, and thanked the stars he had finally gone to get Sci’s opinion. He only regretted not doing it sooner.

The pace began to falter, Cross moving slower but harder. Error moaned, voice breaking as his entrance was stretched around the swollen knot for the first time. It took a few tries, but it finally popped in, locking them together. Cross grunted and sank his teeth in Error’s shoulder, shoving them both over the edge. Error cried out as he was filled with hot seed, abdomen blooming with the purple of Cross’ magic. He came around Cross’ cock, an explosion of color covering his vision. 

He was carefully lowered onto his side, Cross curled up behind him, still locked together. Hands came around to rest on his stomach. Cross pet the soft pseudo flesh there, craning his skull over Error’s shoulder to admire the sight. Error sighed and leaned back into Cross’ arms.

“Do you think it’ll take?” Cross asked after a moment. 

“I don’t know.” Error placed one of his hands over Cross’. “If it doesn’t, my heat will last for several days. And even if it doesn’t then, there will be other heats.”

“And other chances,” Cross agreed. He kissed Error’s shoulder lightly. “I love you, Error. Thank you.”

Error closed his sockets. “I love you too,” he admitted softly. A purr started up against his back and he smiled as his alpha sang to him in the most primal of ways. 

**Author's Note:**

> Error didn’t get pregnant this time, but I can’t say the same about the next time. ;)


End file.
